1. Field of the Invention
The present invention relates to a method of and apparatus for compressing image data of a color image in which the image data are compressed for each pixel block to reduce the amount of the image data.
2. Description of Prior Art
In general, images can be expressed by image data in image processing technique, and the amount of the image data is quite large. Accordingly, compression of the image data is employed for easy storage and transmission. However, the quality of images reproduced from the compressed image data is inferior to that of original images, and the deterioration of the image quality is in proportion of the compression rate of the image data.
Images can be classified into two types. The first type is such that the deterioration of image quality by data compression is visually noticeable. Images of objects the correct shapes of which can be easily remembered, e.g., images of characters and line segments, are typical ones of the first type images. On the other hand, the second type is such that the deterioration of image quality by data compression is hardly noticed, and it includes images of objects the correct shapes of which are not easily remembered. For example, images composed of fine color patterns are typical of the second type images.
In a conventional method of compressing images, the image data are compressed without distinction between these two types of images. Therefore, the deterioration of the image quality with respect to the first type becomes excessive at high compression rates. On the other hand, when the compression rate is lowered, the image quality is improved, however, the compressed image data cannot be stored and transmitted easily.